The exemplary embodiment relates to the use of color patch codes in a printed image for conveying information about the content of the image. In particular, it relates to patch codes for color calibration job identification encoding, although it is to be appreciated that the exemplary embodiment has other applications.
Typically, there are two phases involved in the color correction of a color printer: calibration and characterization. Calibration involves maintaining the printer at a defined state, while characterization involves determination of the multidimensional transformation or profile that relates device color signals (e.g., CMYK) to spectrophotometric or calorimetric signals (e.g. CIELAB). Typically, characterization is performed relatively infrequently, sometimes only once at the factory that produces the printer. Calibration, on the other hand, needs to be performed frequently to compensate for printer drift and bring the device back to its nominal state. While the term calibration will be used throughout this application, the ideas also apply equally to the characterization process.
The calibration process typically involves sending an image with pre-specified device signals (i.e., a target) to the printer, and making spectrophotometric measurements of the print with the use of a sensor, which may be a spectrophotometer, or a scanner, such as an RGB desktop scanner, or any other device capable of sensing color signals at specified locations on the page. The device and spectrophotometric signals together are used to build or update the calibration tables.
When more than one page is used in color calibration, such as when calibrating multiple printers, or using multiple calibration sheets per printer, there is the opportunity for human error in which the pages get mixed up and the wrong calibration(s) result. Patch codes have been developed as a means for providing automation to the color printer calibration process, allowing job identification data and other information to be encoded according to a scheme in which the data is printed according to a protocol and in a format approximately identical to a format of the calibration target. Typically one bit per CMY separation is encoded per patch. Even with only one bit per separation, variations between printer models are sufficient to cause occasional read errors. One solution is to reduce the number of colors further e.g., by eliminating blue, which is too similar to black on some printers, or hand tune the colors to be more distinct. As a consequence, more patches are required to convey the information, occupying a significant portion of the page area.